With machine tool spindles a clamping device (tool clamp) is often provided within the hollow machine tool spindle. This clamping device has a pull-rod extending coaxially relative to the spindle axis, with its front end provided with a gripping collet for engaging the tool in the region of the tool holder. The inner end of the pull-rod passes through a hydraulically acting, so-called release unit, which serves to actuate the clamping device. When the machine tool spindle is stationary, hydraulic oil under high pressure can be fed to the release unit through the first supply channel, the first sliding ring sealing device and the parallel second through-flow channel, whereby the release unit shifts the pull-rod in the machine tool spindle and the gripping collet releases the tool. In order that a cooling lubricant can be fed to the rotating tool, the pull-rod is made hollow and its end projecting out of the release unit is in communication with the central through-flow channel in the hollow shaft. The rotary coupling allows the feed of the cooling lubricant from the stationary housing to the rotating pull-rod through the second supply channel. When changing a tool, i.e. with the machine tool spindle stationary, air can be fed through the rotary coupling and the pull-rod to blow out the tool holder.
Rotary couplings for two different fluids are known in the most varied designs and for the most varied purposes. One rotary coupling of the kind initially described (in-house state of the art) is indeed suitable for wet machining, in which cooling lubricant is constantly fed to the rotating tool, but significant problems arise in dry machining. Since the sliding rings of the second sliding ring sealing device would be rapidly destroyed at high spindle speeds with no lubrication and no cooling with dry running, the second sliding ring sealing device must be supplied with cooling lubricant during dry running also. In order that this cooling lubricant shall not reach the tool during the dry machining, a spring-loaded check valve is fitted in the pull-rod or in the hollow shaft, which valve shuts off the through-flow channel in the hollow shaft or the hollow pull-rod during dry machining, in which the cooling lubricant is fed under low pressure to the rotary coupling. However this non-return valve leads to further problems, since it hinders inter alia the through-flow of the cooling lubricant during wet machining. Furthermore the sliding rings of the second sliding ring sealing device are only cooled inadequately with the use of a non-return valve, since no forced circulation of the cooling lubricant is provided for. For this reason the known rotary coupling is not suitable for higher spindle speeds and increased wear of the sliding rings occurs during dry machining on account of the inadequate cooling. If wet and dry machining alternate, the blowing out of the cooling lubricant from the through-flow channel and the pull-rod on each tool change is extremely troublesome. The workpieces or measuring tools to be kept dry are actually wetted with cooling lubricant. Moreover the operator is affected by fluid mist. In this it is to be noted that, even with continuous dry machining, cooling lubricant must be fed in afresh after each tool change and each blow-out of the second sliding ring sealing device.
Also known are rotary couplings (DE-C1 3 542 014) in which one sliding ring of the sliding ring sealing device can be lifted off the other sliding ring for dry machining. With sliding ring sealing devices the sealing surfaces are very flat and they are very hard. The presence of small solid particles in the cooling lubricant cannot be avoided. With the sliding rings raised solid particles get between the sealing faces and, when the sliding rings are pressed together again, the solid particles lead to rapid destruction of the sliding ring seal. Moreover, with prolonged dry running, a layer can form on the sliding rings which later, when the sliding rings are pressed together again, leads to increased leakage.